1. Field of the Invention
This invention relates to a multi-polar anisotropic resinous magnet to be utilized for small, high performance motor, etc. and a process for producing the same.
2. Related Background Art
A resinous magnet having a resin containing magnetic powder molded into a ring shape, etc. and a large number of anisotropic magnetic poles provided at the peripheral surface thereof has been utilized or its utilization investigated in various fields, determining that it can easily used for a part constituted of a magnetic member such as motor, etc. to make motor, etc. compact and lightweight.
Corresponding to the expansion of the application scope of resinous magnet, there have been made various attempts made to obtain high performance resinous magnets having high magnetic energy and sharp magnetic poles.
For example, Japanese Patent Publication No. 56-5045, etc. discloses a process for preparing a multi-polar anisotropic magnet by injection molding a thermoplastic resin containing magnetic powder to a high density of 80% by weight to 90% by weight in a magnetic field for orientation (anisotropy formation) of magnetic powder and imparting magnetism to the molded product.
The magnetic force in the resinous magnet obtained by use of a resinous material containing magnetic powder is determined primarily by the amount and the magnetic strength of the magnetic powder used for forming the magnetic force effective region. The magnetic force effective region is the region where magnetic powder is oriented by the magnetic field treatment having the same magnetic direction as the magnetic circuit formed by imparting magnetism.
Whereas, in the resinous magnet described in the above-mentioned Japanese Patent Publication No. 56-5045, because of high magnetic powder content, magnetic powder will be contained at high concentration also in the portion which will not become the magnetic force effective region as shown by 1'. If the magnetic powder content is high, interference between magnetic powder such as agglomeration mutually between magnetic powder becomes greater, whereby movements will be mutually obstructed and the powder cannot be sufficiently attracted in the pole-oriented magnetic field toward the magnetic force effective area near the magnet surface. This tendency will be further worsened by increase of the number of poles of the magnet, because the pole-oriented magnetic field will be migrated toward the magnet surface. In other words, the magnetic powder contributing nothing to the magnetic strength of the resinous magnet will be contained at a high concentration in the non-magnetic force effective region, and the resinous magnet obtained according to this method still had problems to be solved from the standpoint of effective utilization of magnetic powder.
Particularly, for obtaining a resinous magnet of higher performance, rare earth magnet powder is frequently used as the magnetic powder, but rare earth magnetic powder is very expensive as compared with ferrite magnet powder. Therefore, when these rare earth magnet powders are utilized, effective utilization of magnetic powder in the resinous magnet is important in lowering the cost of a high performance resinous magnet.
Further, if the content of magnetic powder can be reduced by more effectively utilizing magnetic powder, it also becomes possible to make the resinous magnet itself lightweight.
As in the prior art as described above, by incorporating magnetic powder in a thermoplastic resin at a high concentration of 80 to 90% by weight, flowability is considerably worsened and therefore high mold temperature, high resin temperature, high injection pressure, high injection speed will be required in injection molding. For this reason, special constitutions requiring increased cost are required to be added to the mold, the molding machine, whereby the mold and the molding machine will be enlarged, which also causes the production cost to be increased. Further, in a mold comprising a mold material conventionally used, the capacity of the mold for corresponding to high pressure molding is limited, and in many cases, it is difficult to obtain a mold with high durability in molding under high pressure, high temperature.
As described above, as the method for solving the problem that the magnetic powder cannot be utilized effectively because of formation of a portion contributing nothing to the magnetic force although containing magnetic powder at the central portion of the resinous magnet, namely the non-magnetic force effective region, there has been known a process for preparing a resinous magnet by inserting or outserting, for example, a block made of a thermosetting resin, etc. into the non-magnetic force effective region.
Whereas according to such insert or outsert molding method, improvements can be brought about with respect to effective utilization of magnetic powder because a block, etc. is inserted or outserted into the region contributing nothing to the magnetic force of magnet. However, a resin containing still 80 wt. % to 90 wt. % of magnetic powder is injected into the mold, and therefore there remain problems with respect to molding machine, capacity of the mold durability, etc. Further, by insert or outsert molding, the mold becomes increasingly complicated and the number of steps are increased.
On the other hand, Japanese Laid-open Patent Application Nos. 53-2814 and 53-141499 disclose a process for obtaining a high performance multi-polar anisotropic resinous magnet by increasing the magnetic powder content in the magnetic pole portion magnetized by a multi-polar orientation magnetic field higher than other portions.
However, according to the process disclosed in these Applications, although the effect can be expected to be exhibited with respect to effective utilization of magnetic powder in preparation of a resinous magnet with small number of poles of 2 poles or 4 poles (outer diameter .phi.30), in preparation of a resinous magnet of small diameter with 6 poles or more conventionally used for small high performance stepping rotor, etc., it is factually impossible to expect the effect as mentioned above. This is because, as the pole number is increased from 4 poles, between the adjoining magnet pair becoming the magnetic circuit which is the principle of polar anisotropic orientation, the portion between the magnetic pole gap becoming the string connecting it at the shortest distance where the magnetic field is applied most strongly will be moved toward surface direction of the outside even to the extent that the pole number is increased relative to the central portion of the cavity. For this reason, the lines of magnetic force will be leaked directly between the magnetic poles at the central portion around the cavity as shown by the arrowhead in FIG. 3, whereby the magnetic powder can be attracted only by the very weak leaked flux defined by the permeance between the gaps of the magnetic poles at the central portion, and therefore the magnetic powder cannot be sufficiently converged to the vicinity of the magnetic poles.